This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-402120, filed Dec. 28, 2000, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a light wavelength measuring apparatus that uses a two-beam interferometer and a measuring method therefor. Particularly, the present invention relates to a light wavelength measuring apparatus that measures a wavelength of an incident light to be measured by using a two-beam interferometer, wherein the wavelength of the subject light is measured with high speed and a method therefor.
2. Description of the Related Art
FIG. 22A shows a configuration of a principal portion of a prior-art light wavelength measuring apparatus which uses a two-beam interferometer, which is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 3-279824.
FIGS. 22B and 22C show waveforms of a reference light R and a subject light S coming out of a beam splitter 1102 in FIG. 22A.
Specifically, a light wavelength measuring apparatus 1101 as shown in FIG. 22A is mainly configured by a two-beam interferometer which comprises: a beam splitter 1102 disposed on an optical path to which a subject light and a reference light having a known wavelength are incident; a fixed mirror 1103 serving as a reflection optical system disposed on an optical path of one of the two beams split by the beam splitter 1102; and a movable mirror 1104 serving as another reflection optical system disposed on an optical path of the other of the two beams split by the beam splitter 1102.
According to the light wavelength measuring apparatus 1101 which uses the two-beam interferometer of the above configuration, the subject light, and the reference light having a known wavelength are introduced into the beam splitter 1102. The beam splitter 1102 splits each of the lights for reflection by the fixed mirror 1103 and the movable mirror 1104.
Then, the beams incident on the fixed mirror 1103 and the movable mirror 1104 are reflected back to the beam splitter 1102, combined together by the beam splitter 1102 into respective single beams, and then come out as the reference light R and the subject light S.
In the above, the movable mirror 1104 is moved by a predetermined distance. This causes, as shown in FIGS. 22B and 22C respectively, a power shift or a shift of sine wave in each of the reference light R and the subject light S that are combined and come out by the beam splitter 1102.
In the above, assuming that an optical path difference between the reference light R and the subject light S (a difference in distance between the two optical paths from the point of splitting by the beam splitter 1102 to the combining thereby) is x, an index of refraction is n, and a wavelength of the incident light is xcex, there is generated an interference light component of cos (nx/xcex) in the combined light power.
In order to obtain the wavelength of the subject light, as shown in FIG. 23, a calculation is performed to obtain a wave number k which crosses a reference level L in the combined light power, for each of the reference light R and the subject light S.
Then, calculating means (not shown) calculates the wavelength xcex of the subject light, based on a ratio of the wave numbers in the reference light R and the subject light S, and on a value of the known wavelength of the reference light as follows:
nx=kxc2x7xcex
xe2x80x83xcex=nx/k 
The result of the calculation is outputted.
According to the light wavelength measuring apparatus 1101 which uses the conventional two-beam interferometer as described above, in order to obtain the wavelength of the subject light, it is necessary to move the movable mirror 1104 by a predetermined distance and to obtain the wave number ratio of the interference fringe between the reference light and the subject light. For this reason, it is only after the movable mirror 1104 has been moved that the measurement result can be obtained.
Therefore, according to the light wavelength measuring apparatus 1101 which uses the conventional two-beam interferometer, a certain time must be used for moving the movable mirror 1104. This poses a problem that it is impossible to shorten a measuring time.
Now, the sine curve shift in the output beam power from the two-beam interferometer occurs not only when the movable mirror 1104 is moved but also when there is a change in the wavelength of the incident light, as shown in FIG. 24.
This relationship can be utilized in order to improve the light wavelength measuring apparatus 1101 which uses the conventional two-beam interferometer, thereby achieving a reduction in the measuring time. Specifically, the movable mirror 1104 is fixed, and change in the interference light power caused by the wavelength change in the incident light is detected, so that an amount of change in the wavelength of the incident light can be measured.
However, as shown in FIG. 24, at a point A on the wavelength, the power decreases in each of the case where the wavelength changes in the short wavelength direction and the case where the wavelength changes in the long wavelength direction.
On the contrary, at a point B on the wavelength, the power increases in each of the case where the wavelength changes in the short wavelength direction and the case where the wavelength changes in the long wavelength direction.
As a result, according to the attempt for the improvement described above, when detecting the power change in the interference beam, associating with the wavelength change of the incident light, it is impossible to know in which direction the wavelength is changed to increase or decrease the wavelength. This poses a problem that the amount of wavelength change in the incident light cannot be measured accurately.
The present invention has been made in consideration to the problems described above, and it is therefore an object of the present invention to provide a light wavelength measuring apparatus capable of performing a quick measurement of the wavelength of the subject light, by using a two-beam interferometer which does not require the movement of the movable mirror when measuring the amount wavelength change of the incident light, is capable of properly measuring the amount of wavelength change of the incident light through real-time measurement of how much and in which of the increasing or decreasing directions the wavelength has changed.
Another object of the present invention is to provide a light wavelength measuring apparatus and a method therefor, capable of solving various problems in the course of achieving the above object.
In order to achieve the above objects, according to a first aspect of the present invention, there is provided a light wavelength measuring apparatus comprising:
a two-beam interferometer which splits an incident light in two optical paths, combines and outputs split lights together again, the two-beam interferometer being configured to generate at least one or more combined light made from two beams having polarization states different from each other;
a polarization state detector which detects a variation in the polarization state of the combined light generated by the two-beam interferometer; and
an electric circuit which calculates a wavelength of the incident light based on the variation in the polarization state of the combined light detected by the polarization state detector,
wherein a difference in length between the two optical paths of the two-beam interferometer of a point of splitting the incident light and a point of combining the split lights is fixed, for a detection of polarization variation in the combined light according to a wavelength variation of the incident light by the polarization state detector.
Further, according to a second aspect of the present invention, there is provided the light wavelength measuring apparatus according to the first aspect, wherein at least one of the two optical paths in the two-beam interferometer, from the point of splitting the incident light to the point of combining the split lights, is provided with an optical element which transforms the incident light into a different polarization state.
Further, according to a third aspect of the present invention, there is provided the light wavelength measuring apparatus according to the first aspect, wherein the two-beam interferometer uses a polarization beam splitter as a beam splitter/combiner which splits the incident light into two, combines and outputs the split lights together again.
Further, according to a fourth aspect of the present invention, there is provided the light wavelength measuring apparatus according to the first aspect, wherein a plurality of optical components used in the two-beam interferometer are disposed in tight abutment together on the optical paths in the two-beam interferometer, from the point of splitting the incident light to the point of combining the split lights, a temperature adjusting mechanism being provided for maintaining the optical components at a constant temperature.
Further, according to a fifth aspect of the present invention, there is provided the light wavelength measuring apparatus according to the first aspect, wherein the two-beam interferometer is supplied with a reference light which has a stabilized wavelength, in generally the same optical path as for the subject light, generally simultaneously with the subject light;
the polarization state detector detects a polarization state for each of the combined light of the subject light coming out of the two-beam interferometer and the combined light of the reference light;
a correction is made to an amount of variation in the optical path length included in the polarization state of the subject light, based on the polarization state of the combined light of the subject light and the combined light of the reference light detected by the polarization state detector.
Further, according to a sixth aspect of the present invention, there is provided the light wavelength measuring apparatus according to the fifth aspect, wherein the two-beam interferometer comprises an actuator which slightly varies the length of at least one of the optical paths,
the actuator is subjected to a feedback control based on the polarization or an interference light component, of one of the combined light of the subject light and the combined light of the reference light coming out of the two-beam interferometer.
Further, according to a seventh aspect of the present invention, there is provided the light wavelength measuring apparatus according to the sixth aspect, further comprises a light absorbing cell which absorbs a light of a specific wavelength, and a reference wavelength light source which outputs as the reference light a beam whose wavelength is locked to the specific wavelength absorbed by the light absorption cell.
Further, according to an eighth aspect of the present invention, there is provided the light wavelength measuring apparatus according to the seventh aspect, wherein the light from the reference wavelength light source is frequency-modulated at a predetermined modulation frequency while being locked at the specific wavelength,
frequency response of a circuit to drive the actuator which provides a variable control on the optical path length is set to a frequency sufficiently lower than the predetermined modulation frequency.
Further, according to a ninth aspect of the present invention, there is provided the light wavelength measuring apparatus according to the seventh aspect, further comprising a photo detector which detects a light coming out of the absorption cell, and a trigger generation circuit which generates a trigger when an output signal from the photo detector or a differential signal of the output signal is identical with a specific level,
wherein the electric circuit outputs a wavelength of the subject light, in synchronization with the trigger generated by the trigger generation circuit.
Further, according to a tenth aspect of the present invention, there is provided the light wavelength measuring apparatus according to the first aspect, wherein the two-beam interferometer comprises an optical path varying mechanism which varies the length of at least one of the two optical paths,
the polarization state detector detects a first amount of variation in the polarization state of the combined light caused when the optical path length varying mechanism varies the optical path length by a predetermined distance, for a detection of an absolute value of wavelength of the incident light, and a second amount of variation in the polarization state of the combined light caused by a wavelength variation in the subject light when the optical path length is fixed, for a detection of polarization condition according to a wavelength variation of the incident light,
the electric circuit performs real-time measurement of an absolute wavelength of the subject light, based on the first and second amounts of variation detected by the polarization state detector.
Further, according to an eleventh aspect of the present invention, there is provided the light wavelength measuring apparatus according to the first aspect, wherein the two-beam interferometer has a light entering portion provided with a double-image polarizing element which separates the incident light into lights of first and second polarization components which are perpendicular to each other.
Further, according to a twelfth aspect of the present invention, there is provided a light wavelength measuring method comprising:
preparing a two-beam interferometer which splits an incident light in two optical paths, combines and outputs the split lights together again, the two-beam interferometer generating at least one combined light made from two beams having polarization states different from each other;
detecting a first amount of variation in the polarization state of the combined light caused by the variation in the optical path length by the predetermined distance;
detecting a second amount of variation in the polarization state of the combined light caused by a wavelength variation in the subject light when an optical path length of at least one of the two optical paths in the two-beam interferometer is fixed;
varying an optical path length of at least one of the two optical paths in the two-beam interferometer, by a predetermined distance; and
performing a real-time calculation of a wavelength of the incident light based on the first and the second amounts of variation.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.